SLUB: support slub_debug on by default

Add a new configuration variable

CONFIG_SLUB_DEBUG_ON

If set then the kernel will be booted by default with slab debugging
switched on. Similar to CONFIG_SLAB_DEBUG. By default slab debugging
is available but must be enabled by specifying "slub_debug" as a
kernel parameter.

Also add support to switch off slab debugging for a kernel that was
built with CONFIG_SLUB_DEBUG_ON. This works by specifying

slub_debug=-

as a kernel parameter.

Dave Jones wanted this feature.
http://marc.info/?l=linux-kernel&m=118072189913045&w=2

[akpm@linux-foundation.org: clean up switch statement]
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 87 insertions, 45 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4344f69ae24a..8916410307f1 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1580,35 +1580,39 @@ and is between 256 and 4096 characters. It is defined in the file
 
         slram=          [HW,MTD]
 
-        slub_debug      [MM, SLUB]
-                        Enabling slub_debug allows one to determine the culprit
-                        if slab objects become corrupted. Enabling slub_debug
-                        creates guard zones around objects and poisons objects
-                        when not in use. Also tracks the last alloc / free.
-                        For more information see Documentation/vm/slub.txt.
+        slub_debug[=options[,slabs]]    [MM, SLUB]
+                        Enabling slub_debug allows one to determine the
+                        culprit if slab objects become corrupted. Enabling
+                        slub_debug can create guard zones around objects and
+                        may poison objects when not in use. Also tracks the
+                        last alloc / free. For more information see
+                        Documentation/vm/slub.txt.
 
         slub_max_order= [MM, SLUB]
-                        Determines the maximum allowed order for slabs. Setting
-                        this too high may cause fragmentation.
-                        For more information see Documentation/vm/slub.txt.
+                        Determines the maximum allowed order for slabs.
+                        A high setting may cause OOMs due to memory
+                        fragmentation. For more information see
+                        Documentation/vm/slub.txt.
 
         slub_min_objects=       [MM, SLUB]
-                        The minimum objects per slab. SLUB will increase the
-                        slab order up to slub_max_order to generate a
-                        sufficiently big slab to satisfy the number of objects.
-                        The higher the number of objects the smaller the overhead
-                        of tracking slabs.
+                        The minimum number of objects per slab. SLUB will
+                        increase the slab order up to slub_max_order to
+                        generate a sufficiently large slab able to contain
+                        the number of objects indicated. The higher the number
+                        of objects the smaller the overhead of tracking slabs
+                        and the less frequently locks need to be acquired.
                         For more information see Documentation/vm/slub.txt.
 
         slub_min_order= [MM, SLUB]
                         Determines the mininum page order for slabs. Must be
-                        lower than slub_max_order
+                        lower than slub_max_order.
                         For more information see Documentation/vm/slub.txt.
 
         slub_nomerge    [MM, SLUB]
-                        Disable merging of slabs of similar size. May be
+                        Disable merging of slabs with similar size. May be
                         necessary if there is some reason to distinguish
-                        allocs to different slabs.
+                        allocs to different slabs. Debug options disable
+                        merging on their own.
                         For more information see Documentation/vm/slub.txt.
 
         smart2=         [HW]
diff --git a/Documentation/vm/slub.txt b/Documentation/vm/slub.txt
index 1523320abd87..df812b03b65d 100644
--- a/Documentation/vm/slub.txt
+++ b/Documentation/vm/slub.txt
@@ -41,6 +41,8 @@ Possible debug options are
         P               Poisoning (object and padding)
         U               User tracking (free and alloc)
         T               Trace (please only use on single slabs)
+        -               Switch all debugging off (useful if the kernel is
+                        configured with CONFIG_SLUB_DEBUG_ON)
 
 F.e. in order to boot just with sanity checks and red zoning one would specify:
 
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index fab32a286371..640844024ffd 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -152,6 +152,19 @@ config DEBUG_SLAB_LEAK
         bool "Memory leak debugging"
         depends on DEBUG_SLAB
 
+config SLUB_DEBUG_ON
+        bool "SLUB debugging on by default"
+        depends on SLUB && SLUB_DEBUG
+        default n
+        help
+          Boot with debugging on by default. SLUB boots by default with
+          the runtime debug capabilities switched off. Enabling this is
+          equivalent to specifying the "slub_debug" parameter on boot.
+          There is no support for more fine grained debug control like
+          possible with slub_debug=xxx. SLUB debugging may be switched
+          off in a kernel built with CONFIG_SLUB_DEBUG_ON by specifying
+          "slub_debug=-".
+
 config DEBUG_PREEMPT
         bool "Debug preemptible kernel"
         depends on DEBUG_KERNEL && PREEMPT && TRACE_IRQFLAGS_SUPPORT
diff --git a/mm/slub.c b/mm/slub.c
index e0cf6213abc0..6aea48942c29 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -323,7 +323,11 @@ static inline int slab_index(void *p, struct kmem_cache *s, void *addr)
 /*
  * Debug settings:
  */
+#ifdef CONFIG_SLUB_DEBUG_ON
+static int slub_debug = DEBUG_DEFAULT_FLAGS;
+#else
 static int slub_debug;
+#endif
 
 static char *slub_debug_slabs;
 
@@ -888,38 +892,57 @@ fail:
 
 static int __init setup_slub_debug(char *str)
 {
-        if (!str || *str != '=')
-                slub_debug = DEBUG_DEFAULT_FLAGS;
-        else {
-                str++;
-                if (*str == 0 || *str == ',')
-                        slub_debug = DEBUG_DEFAULT_FLAGS;
-                else
-                for( ;*str && *str != ','; str++)
-                        switch (*str) {
-                        case 'f' : case 'F' :
-                                slub_debug |= SLAB_DEBUG_FREE;
-                                break;
-                        case 'z' : case 'Z' :
-                                slub_debug |= SLAB_RED_ZONE;
-                                break;
-                        case 'p' : case 'P' :
-                                slub_debug |= SLAB_POISON;
-                                break;
-                        case 'u' : case 'U' :
-                                slub_debug |= SLAB_STORE_USER;
-                                break;
-                        case 't' : case 'T' :
-                                slub_debug |= SLAB_TRACE;
-                                break;
-                        default:
-                                printk(KERN_ERR "slub_debug option '%c' "
-                                        "unknown. skipped\n",*str);
-                        }
+        slub_debug = DEBUG_DEFAULT_FLAGS;
+        if (*str++ != '=' || !*str)
+                /*
+                 * No options specified. Switch on full debugging.
+                 */
+                goto out;
+
+        if (*str == ',')
+                /*
+                 * No options but restriction on slabs. This means full
+                 * debugging for slabs matching a pattern.
+                 */
+                goto check_slabs;
+
+        slub_debug = 0;
+        if (*str == '-')
+                /*
+                 * Switch off all debugging measures.
+                 */
+                goto out;
+
+        /*
+         * Determine which debug features should be switched on
+         */
+        for ( ;*str && *str != ','; str++) {
+                switch (tolower(*str)) {
+                case 'f':
+                        slub_debug |= SLAB_DEBUG_FREE;
+                        break;
+                case 'z':
+                        slub_debug |= SLAB_RED_ZONE;
+                        break;
+                case 'p':
+                        slub_debug |= SLAB_POISON;
+                        break;
+                case 'u':
+                        slub_debug |= SLAB_STORE_USER;
+                        break;
+                case 't':
+                        slub_debug |= SLAB_TRACE;
+                        break;
+                default:
+                        printk(KERN_ERR "slub_debug option '%c' "
+                                "unknown. skipped\n",*str);
+                }
         }
 
+check_slabs:
         if (*str == ',')
                 slub_debug_slabs = str + 1;
+out:
         return 1;
 }